Size-Scalable Near-Infrared Photoluminescence in Gold Monolayer Protected Clusters

Near-infrared photoluminescence of a series of three gold monolayer protected clusters (MPCs) with volumes spanning 50-200 ų was studied by using variable-temperature photoluminescence (VT-PL) spectroscopy. The three MPCs, which included Au₂₀(SC₈H₉)₁₅-diglyme, Au₂₅(SC₈H₉)₁₈, and Au₃₈(SC₁₂H₂₅)₂₄, all exhibited temperature-dependent intensities that reflected a few-millielectronvolt energy gap that separated bright emissive and dark nonradiative electronic states. All clusters showed increased PL intensities upon raising the sample temperature from 4.5 K to a cluster-specific value, upon which increased sample temperature resulted in emission quenching. The increased PL in the low-temperature range is attributed to thermally activated carrier transfer from dark to bright states. The quenching at elevated temperatures is attributed to nonradiative vibrational relaxation through Au-Au stretching of the MPCs metal core. Importantly, the results show evidence of a common and size scalable metal-centered intraband PL mechanism that is general for ultrasmall metal nanoclusters, which are expected to show nonscalable optical properties.